On 2 Aug, 12:29, Rob wrote:
With access from the inside, and about a 3mm view of the 'bolt' between
the striking plate and the door, what's the best and least destructive
way of removing this (lost key) external door lock?

Angle grinder!

Thinnest cutting disk you can get (Aldi's stainless steel ones are
great for this) and go through the bolt, leaving the lock body
unharmed. You can even swap the bolt afterwards to repair it and leave
the keying unchanged.

If the bolt is just brass, not with hardened rollers, then a Fein
Mutleymaster will do it too.